This invention relates to ultrasonic transmission apparatus and, more particularly, to such apparatus which transmits ultrasonic energy from a source to a distal tip with minimal loss, and which is particularly adaptable for medical applications.
The field of balloon angioplasty provides an established technique for reducing vascular obstructions caused by thrombi and plaque deposits. Here, a catheter having an inflatable balloon at its distal end is inserted into a patient's blood vessel and then, by use of a guide wire, in cooperation with an observation system, the catheter is advanced until it reaches the obstruction (e.g. a thrombus) in question. Then, the balloon is inflated with the hope of reducing the obstruction. Unfortunately, balloon angioplasty, although offering a desirable alternative to arterial bypass surgery, suffers significant drawbacks. For example, the procedure is neither effective nor safe in cases of thrombus. Thrombus often is not destroyed by the inflated balloon, thus resulting in relatively quick re-occlusion. In addition, balloon angioplasty often is accompanied by significant damage to the blood vessel which further stimulates thrombus formation and re-occlusion.
Other catheter-based procedures have been proposed as alternatives to bypass surgery, such as laser-type angioplasty, mechanical drills and, most recently, ultrasonic angioplasty. One example of ultrasonic angioplasty apparatus is described in copending application Ser. No. 449,465, assigned to the same assignee as the present invention.
In a typical ultrasonic angioplasty device, a long, thin ultrasonic transmitter connects a tip at its distal end to a power source at its proximal end. Using standard angioplasty techniques, this transmitter is inserted into and guided through the patient's blood vessel until the distal tip arrives at the occlusion. Then, energization of the power source produces ultrasonic displacement that is transmitted to the tip, resulting in destruction of the thrombus. However, and as found in the ultrasonic angioplasty apparatus described in U.S. Pat. No. 4,870,953, the transmission of ultrasonic energy through the ultrasonic transmitter could generate an inordinate amount of heat which, if not removed, could result in serious damage to the patient's blood vessels. Accordingly, the apparatus described in U.S. Pat. No. 4,870,953 provides a cooling arrangement in which the ultrasonic transmitter is disposed in a cooling bath, namely a catheter that is flushed with a physiologic solution to cool the entire transmitter.
It has been found that heat generation is common to most materials heretofore used for ultrasonic angioplasty because those materials produce significant attenuation of the ultrasonic energy. Consequently, acoustic energy is transformed to thermal energy. For the purpose of coronary procedures, the ultrasonic energy must be transmitted over a distance on the order of about 125-150 cm.; and the attenuation presented by this length of material requires an extremely high input energy level in order for sufficient ultrasonic displacement to be produced at the tip. Therefore, the heat generated by the typical ultrasonic angioplasty device increases the probability of material fatigue which may result in fracture of the device while in use.
The aforementioned patent application 449,465 is directed to a novel arrangement which overcomes these drawbacks, disadvantages and hazards. As disclosed therein, the ultrasonic transmitter is formed of material having a high mechanical Q, thus minimizing the attenuation experienced by the ultrasonic energy as it is transmitted through this transmitter and thereby minimizing heat generation. Preferably, aluminum or an aluminum alloy having a mechanical Q greater than 50,000 is used. Examples of suitable alloys include duralumin, hiduminium, AL-7075, AL-2024 and AL-6061. The generation of heat is substantially obviated; and it no longer is necessary to use an ultrasonic source of high energy levels in order to drive the transmitter.
While the aforementioned ultrasonic angioplasty device obtains benefits and results not previously realized, further investigation into ultrasonic angioplasty has led to certain observations, culminating in the invention disclosed herein.
It has been found that the cross-sectional area of the ultrasonic transmitter directly affects the attenuation of the ultrasonic energy transmitted thereby. That is, a greater cross sectional diameter results in less attenuation of the transmitted ultrasonic energy, thereby permitting the use of an ultrasonic energy source having a lower energy level. But, an ultrasonic transmitter of greater cross-sectional diameter results in a more rigid transmission member which may not be able to follow easily the bends inherent in typical blood vessels.
It also has been found that an ultrasonic transmitter of reduced cross sectional diameter formed of high mechanical Q material may be susceptible to easy fracture or fatigue. Thus, although a very thin ultrasonic transmitter may exhibit 14 sufficient flexibility, it also presents an extremely high risk of breakage due to fatigue and to significant bending thereof as it follows a blood vessel.
It has been observed, that, when a physician uses a typical ultrasonic angioplasty device, he manually guides it into the patient's blood vessel and, more often than not, grasps a portion of the transmission member while ultrasonic energy is transmitted therethrough. This presents a problem because it results in substantial damping of ultrasonic displacement, thereby seriously reducing the operating efficiency of the device.
Although many conventional ultrasonic medical instruments, such as an ultrasonic scalpel, operate at frequencies in the range 20-30 kHz, it has been found that such frequencies do not permit maximum displacement at the tip of the apparatus when the device is bent. However, the higher frequencies needed for more optimum displacement present more difficult design parameters, they result in greater attenuation of the transmitted ultrasonic energy and, for the same displacement, they produce greater internal stress which increases the tendency of the transmitter to fracture due to fatigue. On the other hand, however, a higher ultrasonic frequency permits the transmitter to be subjected to a sharper bend without as significant an energy loss as at lower frequencies and, thus, the use of such higher frequencies in an ultrasonic angioplasty device permits that device to be used in blood vessels and lumens having tighter turns.
It also has been observed, that since patient safety is of the highest priority, care must be taken in the design of the ultrasonic angioplasty device to minimize hazards and risk of injury to the patient in the event of a malfunction or break in the device.